Dynamaxing happens to be the newest mechanic introduced in Pokemon Sword and Shield. Inspired by Kaiju-style monsters, it transforms Pokemon into giant building-sized creatures while dramatically boosting their stats. Usually, the most effective method to combat Dynamaxing involves a trainer Dynamaxing their own Pokemon in response, an obvious choice, but not the most optimal one.
Dynamaxing in response to an opponent's Dynamax in Pokemon Sword and Shield typically means that a trainer had to use their trump card before they wanted to, which can throw off any strategies they had for later on in the battle. It doesn't help that the transformation has immunity to nearly everything that could cancel it - but only nearly.
Pokemon trainer WolfeVGC found a way to shut down Dynamaxing the moment an opponent triggers it. Game Freak may have made the transformation immune to flinching and phasing attacks (things like Whirlwind and Roar), but it seems to have missed the Pokemon item Eject Button. Eject Button is a held, consumable item that forces the holder to switch places with another Pokemon in the party. The idea is to get the Dynamaxing Pokemon in question to hold the item, and then damage them, which will force them back into the opponent's party, effectively wasting the opponent's only Dynamax.
Trainers looking to maximize the effectiveness of this method will need to perform it in a double battle. They will need a Pokemon that knows Trick, and another Pokemon that knows Quick Attack. The Pokemon that knows Trick will also need to be the one holding Eject Button as Trick switches the items held between the user and the target. After Trick activates, Quick Attack will follow, damage the Dynamaxed Pokemon, trigger Eject Button, and send the Dynamax back into its ball, neutralizing the transformation. Of course, this also works on any Gigantamaxed Pokemon, too.
The strategy does require the user to have a prediction of what Pokemon their opponent will Dynamax, though, and can be a bit gimmicky. Still, pulling it off is one of the most satisfying things in Sword and Shield as the Dynamax mechanic can be pretty overwhelming. The Pokemon community's opinions of it in competitive play are actually pretty divided. Smogon, a large, prominent, fan-run Pokemon forum, outright banned it from their competitive events due to how easy it can steamroll an opposing players' team. WolfeVGC made this strategy as a way to poke fun at the situation.
Pokemon Sword and Shield are now out for the Nintendo Switch.